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Racing Ahead

StarDate:

March 16, 2012

The motion of the planets around the Sun is like a never-ending race. Each planet follows its own lane around the Sun, and moves at its own speed. And as you might expect, a planet in an inside lane always beats one in an outside lane.

Consider Earth and Mars.

Earth is the third planet out from the Sun. Our “lane” -- our path around the Sun -- is close to 600 million miles long. It takes us one year to complete one lap around the track, for an average speed of 67,000 miles per hour.

Mars is half-again as far out from the Sun, so its lane is longer than Earth’s. A body in a farther orbit moves more slowly, so Mars is moving about 13,000 miles per hour slower than Earth is, so it takes almost twice as long to finish a lap around the Sun.

As a result, the viewing angle to Mars is constantly changing.

Earth passed Mars early this month. As we approached the planet, it appeared to slow down, then reverse direction against the background of stars. The same thing would happen if you passed another car on the highway -- or the racetrack.

As we passed Mars, we were closest to it for the year, so the planet was at its brightest. And it’s still quite bright, shining like an orange star in the east as night falls.

Over the coming weeks and months, though, as the distance to Mars increases, it’ll fade rapidly. By late summer, it’ll be just one-eighth as bright as it is now. So look for Mars at its best -- before we leave the Red Planet behind.

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